WO2019076122A1

WO2019076122A1 - Lithium battery cathode material, preparation method thereof, and lithium battery using the cathode material

Info

Publication number: WO2019076122A1
Application number: PCT/CN2018/100850
Authority: WO
Inventors: Liu Li; Qiming Pan; Hui Chen; Zhaojun LUO
Original assignee: Shenzhen Bak Power Battery Co., Ltd.
Priority date: 2017-10-19
Filing date: 2018-08-16
Publication date: 2019-04-25
Also published as: CN107768639A; TW201917938A; TWI678835B

Abstract

A lithium ion battery cathode material and a synthesis method thereof are provided. The cathode material is spherical secondary particles comprised of primary particles with diameter from 100 to 200 nm. The secondary particles have porous structure with a pore diameter of 1 to 2 μm. The cathode material has the chemical formula of Li 1+xM 1-xO 2, where x=0.05-0.25, M is one or more selected from the group consisting of Ni, Co, Mn, Al, Mg, Fe, B, Ti, Cr, Ga, Zn, V, Ge, and Sn. The lithium ion battery cathode material has high energy density, high electrochemical activity, and can well withstand the stress and strain during charging and discharging. A method for preparing this cathode material is also provided. The method not only increases the degree of homogeneity of raw material mixing, but also increases the activity of the cathode material, thus macroscopically improving the cycling stability of the lithium ion battery cathode material during charging and discharging.

Description

LITHIUM BATTERY CATHODE MATERIAL, PREPARATION METHOD THEREOF, AND LITHIUM BATTERY USING THE CATHODE MATERIAL

Technical Field

The present invention belongs to the field of lithium ion battery manufacturing, and particularly relates to a lithium ion battery cathode material and a preparation method thereof, and to a lithium ion battery using this cathode material as a battery cathode.

Background of the Invention

Compared with traditional secondary batteries, lithium ion batteries ("lithium batteries" for short) have the advantages of high platform voltage (up to 3.2-3.7 V) , high energy density, no memory effect, etc., and thus find wide application in electronic products such as smartphones, cameras, computers, etc. Lithium ion secondary batteries are not only used in the field of 3C products, but also widely employed in industrial fields such as electric vehicles and uninterruptible power supplies, etc. A lithium ion battery is mainly composed of a cathode material of a lithium-metal oxide, a liquid organic electrolyte or a solid electrolyte, and an anode material. The performance of the cathode material, as an important component of a lithium ion battery, is a major factor affecting the quality of the battery. Given the fact that lithium ion batteries in the prior art generally have an specific anode capacity higher than 300 mAh/g, how to improve the performance of the cathode material becomes an important research direction for improving the performance of lithium ion batteries.

In the prior art, commercially available lithium ion batteries mainly employ LiMO ₂ as cathode material, wherein M is a metal ion such as Ni, Mn, Co, or Al. LiMO ₂ is a layered cathode material having a very high theoretical capacity and can meet the requirement of high-energy-density lithium ion batteries. This material can be prepared by a simple solid-state sintering method with low cost and simple preparation process. Although LiMO ₂ has high capacity and stability, it is still not ideal in terms of environmental and economic friendness. The most important affecting factors are as follows:

1. In preparing the LiMO ₂ layered cathode material by the solid phase sintering method, it is difficult to homogeneously mix the raw materials, resulting in segregation of the metal ions in the raw materials that leads to ion mixing. The mixed metal ions in the lithium layer would hinder the transport of lithium ions, thereby reducing the specific capacity of the cathode material.

2. The secondary particles of LiMO ₂ cathode material prepared by the traditional solid phase sintering method are relatively dense and have a low specific surface area. This cathode material would have limited contact area with the electrolyte in battery, resulting in low electrochemical activity, which compromises the actual specific capacity.

3. The LiMO ₂ cathode material particles prepared by the traditional method are susceptible to the stress and strain resulting from lithium ion intercalation and deintercalation in the charge and discharge cycles. Hence, the particles cannot withstand the strong stress and strain caused by lithium ion intercalation and deintercalation in the battery and are prone to cracking. The cracked particles fail to achieve continuous transport of lithium ions and electrons, resulting in an increase in internal resistance of the battery and a decrease in battery capacity.

Although the above problems have been confirmed in many literatures, they have not been effectively addressed in the actual development of lithium ion battery cathode materials. The reason for this is as follows. Presently, the bottleneck in the application of lithium ion batteries still mainly lies in energy density. The efficiency of lithium ion batteries will be improved as long as the energy density is increased. Theoretically, a cathode material with denser particles can increase the energy density. However, the denser the particles of the cathode material, the more significant the problems described above associated with low specific surface area of the particles and susceptibility of the particles to the stress and strain resulting from charging and discharging, leading to no meaningful progress in the development of lithium ion cathode materials.

Summary of the Invention

In order to address the drawbacks of the prior art, the present invention proposes an alternative method for preparing a cathode material. In the same manner as in the prior art in which the energy density of lithium ion battery cathode materials is improved, the present invention also employs the solid phase sintering method to prepare a lithium ion battery cathode material, such that the cathode material prepared has sperical particles and high energy density. Moreover, the preparation method of the present invention involves adding a volatile organic acid and an organic polymer prior to the solid phase sintering process. This not only increases the degree of homogeneity of raw material mixing, such that the possibility of segregation of the metal ions in the raw materials is greatly reduced and the degree of ion mixing in the product is decreased, but also allows uniform porous structure to be formed from decomposition of the organic polymer during the sintering process. The uniform porous structure not only provides channels for electrolyte infiltration, enhancing the activity of the cathode material, but also serves as a buffer area for the cathode material to cope with the stress and strain during lithium ion intercalation and deintercalation, thus macroscopically improving the cycling stability of the lithium ion battery during charging and discharging.

The technical effects to be achieved by the present invention are effected by the following technical solutions.

The lithium ion battery cathode material of the present invention is of a lithium oxide. The cathode material is composed of spherical secondary particles comprised of primary particles with diameter from 100 to 200 nm. The secondary particles have porous structure therein the pore diameter is 1 to 2 μm.

The lithium-ion-battery used cathode material provided in the present invention has the chemical formula of Li _1+xM _1-xO ₂, wherein x=0.05-0.25, M is one or more selected from the group consisting of Ni, Co, Mn, Al, Mg, Fe, B, Ti, Cr, Ga, Zn, V, Ge, and Sn.

Compared with the prior art cathode material with dense particles and thus theoretically high energy density, the present invention employs lithium-metal oxide having a porous structure as the cathode material. The pore structures in the cathode material not only provide channels for electrolyte infiltration, enhancing the activity of the cathode material, but also serve as buffer area for the cathode material to cope with the stress and strain during lithium ion deintercalation, thus macroscopically improving the cycling stability of the lithium ion battery cathode material during charging and discharging.

The present invention also provides a method for preparing a lithium ion battery cathode material, comprising the following steps:

S01: Mixing a metal salt and a lithium salt in a molar ratio of metal ion to lithium ion of 1: (1-1.3) , adding a volatile organic acid in an amount sufficient for complexing all of the metal ions in the metal salt and the lithium salt in the raw materials, and ball-mixing and grinding the metal salt, the lithium salt and the volatile organic acid to obtain homogeneity.

In the sintering process of the prior art, it is generally not possible to obtain a completely homogeneous mixture of the metal salt and the lithium salt by physical agitation or dispersion, which results in the uniformity of the product after sintering being limited by the effect of the dispersion process. In the present invention, a volatile organic acid is added during mixing the salts. The volatile organic acid can effectively complex the metal ions in the metal salt and the lithium ions in the lithium salt, so that the salts are not only mechanically mixed but also more tightly bonded by cordinatoin bonding force.

Since the volatile organic acid is volatile and produces no residue, there is no particular limitation on the amount of the volatile organic acid added, so long as it can complex all of the metal ions in the metal salt and the lithium salt in the raw materials. In actual use, it can be added in excess.

S02: Mixing the ground product from S01 with an organic polymer in a ratio of 1 to 3 mL of the organic polymer to 1 g of the ground product, and grinding the mixture homogeneously.

The addition of the organic polymer allows the metal salt and the lithium salt to be mixed further homogeneously, thus enhancing the degree of homogeneity of raw material mixing, reducing the possibility of ion segregation and mixing in the product. More importantly, the organic polymer is burned out under high temperature conditions during subsequent sintering, such that the organic matter decomposes to generate gas during the sintering process, thereby forming pore structures in the microstructure of the cathode material in the present invention. In this premise, it is necessary to select a suitable organic polymer. Those organic polymers that generate excessive gases or produce ash residues during burning may not be used. In the present invention, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 2000, polyethylene glycol 6000, or polyethylene glycol 12000, or a combination of two or more thereof, is preferably used.

S03: Pre-sintering the ground product from S02 at 300-500℃ for 4 to 10 hours.

S04: Sintering the pre-sintered product from S03 at 750-950℃ for 6 to 15 hours, and cooling the product to obtain the desired lithium ion battery cathode material.

Compared with the prior art, the invention enhances the homogeneity of raw material mixing by adding a volatile organic acid and a liquid organic polymer in the synthesis process, thereby reducing the possibility of ion segregation, reducing the degree of ion mixing in the product, and increasing the speed of lithium ion transport. Moreover, the polymer decomposes, leaving uniform pore structures. Such pore structures can on the one hand provide channels for electrolyte infiltration, thus enhancing the electrochemical activity of the material, and on the other hand buffer the stress and strain resulting from intercalation and deintercalation of lithium ions during charging and discharging, thus maintaining structural integrity and stabilizing the cycling performance of the cathode material.

The grinding time in S01 and S02 depends on the amount of the raw materials and generally ranges from 0.5 to 10 h. The more the raw materials, the longer the grinding time. The grinding is conducted such that the raw materials are mixed homogeneously and the particle size of the raw materials is uniform. Similarly, the pre-sintering time and the sintering time can be adjusted according to the difficulty in decomposition of the salts and the total amount of the raw materials to be sintered. The more difficult the decomposition of the salts and the larger the total amount of the raw materials to be sintered, the longer the sintering time.

Further, in S03, the pre-sintering temperature is 380 to 420℃; and in S04, the sintering temperature is 800 to 850℃.

The present invention also provides a lithium ion battery using the above-mentioned cathode material as a battery cathode material, wherein the lithium ion battery has a capacity retention rate of higher than 94%after 50 cycles of testing, and of higher than 88%after 100 cycles of testing, under the test condition of a cycling test current density of 0.05-5C.

The present invention has the following advantages:

1. The lithium ion battery cathode material of the present invention has high density, high activity, and can well withstand the stress and strain during charging and discharging.

2. The preparation method for a lithium ion battery cathode material according to the present invention involves adding a volatile organic acid and an organic polymer prior to the solid phase sintering process. This not only increases the degree of homogeneity of raw material mixing, such that the possibility of segregation of the metal ions in the raw materials is greatly reduced and the degree of ion mixing in the product is decreased, but also allows uniform pore structures to be formed from decomposition of the organic polymer during the sintering process. The uniform pore structures not only provide channels for electrolyte infiltration, thus enhancing the activity of the cathode material, but also constitute a buffer area for the cathode material to cope with the stress and strain during lithium ion deintercalation, thus macroscopically improving the cycling stability of the lithium ion battery cathode material during charging and discharging.

Brief Description of the Drawings

Fig. 1 shows the XRD scan patterns for the lithium ion battery cathode material in Example 1 versus that in Comparative example 1 of the present invention;

Fig. 2 shows the lithium ion battery cathode material in Example 1 versus that in Comparative example 1 of the present invention obtained under a field emission scanning electron microscope (S-4300 Shimadzu, 15 kV) , wherein Figs. 2a-b represent the particle morphology of the material in Comparative example 1, and Figs. 2c-f represents the particle morphology of the material in Example 1;

Fig. 3 shows the first charge and discharge curves for the batteries made of the cathode material in Example 1 versus that in Comparative example 1 of the present invention;

Fig. 4 shows the capacity differential curves corresponding to the first charge and discharge curves for the batteries made of the cathode material in Example 1 versus that in Comparative example 1 of the present invention;

Fig. 5 shows the curves of cycling performance test (test condition 0.2C) for the batteries made of the cathode material in Example 1 and Example 5 versus that in Comparative example 1 of the present invention;

Fig. 6 shows the curves of cycling performance test (test condition 0.5C) for the batteries made of the lithium ion battery cathode material in Example 1 and Example 5 versus that in Comparative example 1 of the present invention;

Fig. 7 shows the results of cycling performance test (test condition 0.5C) for the batteries made of the cathode material in Example 6 and Example 7 versus that in Comparative example 1 of the present invention;

Fig. 8 shows the results of cycling performance test (test condition 0.5C) for the batteries made of the cathode material in Example 8 and Example 9 versus that in Comparative example 1 of the present invention;

Fig. 9 shows the results of cycling performance test (test condition 0.5C) for the batteries made of the cathode material in Examples 10, 11, 12 and 5 versus that in Comparative example 1 of the present invention;

Fig. 10 shows the microscopic feature of the cathode material particles after 50 cycles at 0.2 C of the batteries made of the cathode material in Example 1 versus that in Comparative example 1 of the present invention.

Detailed Description

The present invention will now be described in detail with reference to drawings and embodiments.

The raw material components and the amounts thereof in mole, gram or volume used in the preparation of the lithium ion battery cathode material in the Examples are set forth in the following table:

The preparation method for the cathode materials in the Examples was as follows:

S01: mixing the metal salt, the lithium salt and the volatile organic acid in the specified ratio, and grinding the mixture for 5 hours;

S02: mixing the ground product from S01 with the organic polymer, and grinding the mixture for 2 hours;

S03: pre-sintering the ground product from S02;

S04: sintering the pre-sintered product from S03, and cooling the product to obtain the desired lithium ion battery cathode material.

The specific parameters for the preparation method are set forth as follows:

Example

Pre-sintering

Sintering

No.	conditions	conditions
1	400℃, 4h	750℃, 6h
2	400℃, 4h	900℃, 10h
3	400℃, 4h	900℃, 12h
4	400℃, 6h	850℃, 15h
5	400℃, 4h	750℃, 15h
6	400℃, 4h	750℃, 15h
7	400℃, 4h	800℃, 15h
8	400℃, 4h	750℃, 15h
9	400℃, 4h	750℃, 15h
10	400℃, 4h	750℃, 15h
11	400℃, 4h	750℃, 15h
12	400℃, 4h	750℃, 15h
13	420℃, 6h	800℃, 12h
14	420℃, 6h	800℃, 12h
15	420℃, 6h	800℃, 12h
16	420℃, 6h	800℃, 12h
17	420℃, 6h	800℃, 12h
18	420℃, 6h	800℃, 12h
19	460℃, 6h	880℃, 10h
20	460℃, 6h	880℃, 10h
21	460℃, 6h	880℃, 10h
22	460℃, 6h	880℃, 10h
23	460℃, 6h	880℃, 10h
24	460℃, 6h	880℃, 10h
25	460℃, 6h	880℃, 10h
26	480℃, 5h	900℃, 10h
27	480℃, 5h	900℃, 10h
28	480℃, 5h	900℃, 10h
29	500℃, 6h	900℃, 10h
30	500℃, 6h	950℃, 8h
Comp. Ex. 1	400℃, 4h	750℃, 6h
Comp. Ex. 2	400℃, 4h	750℃, 6h

A CR2016 button cell was made using the respective cathode materials prepared, wherein the cathode was made of 80 wt%of the cathode material, 10 wt%of pvdf and 10 wt%of acetylene black, the counter electrode was a lithium sheet, the separator was Celgard 2500, and the electrolyte was an organic solution of 1 M of LiPF ₆ in EC: DEC=1: 1. The fresh battery was charged at a current of 0.2 C at 25℃ until the voltage was 4.3 V, and the charged battery was discharged at a current of 0.2C until the voltage was 2.8 V, the discharge capacity being recorded as the first discharge capacity. The button cells made were cycled for 50 and 100 cycles respectively under the conditions of 0.2C/0.2C and 0.5C/0.5C. The test results are as follows:

Fig. 1 shows the XRD scan patterns for the cathode material in Example 1 versus that in Comparative example 1. The crystal lattice structure of the powder of the materials was analyzed using a Shimadzu XRD-6000 X-ray diffractometer (Cu-Kαradiation, λ = 1.5418

) at a 2θ scanning angle of 10-80° and a scanning speed is 1°/min. It can be seen from Fig. 1 that both materials had a similar R-3m structure, but had a different ratio of intensity of (003) peak to that of (104) peak, indicating that the ion mixing was different. The higher the ratio of I (003) /I (104) , the less ion mixing in the cathode material. It can be calculated from Fig. 1 that the I (003) /I (104) was 1.346 in Example 1 (PA) , and the I (003) /I (104) was 1.207 in Comparative example 1 (SS) . This result indicates that the material obtained in this example had a low degree of ion mixing and thus a higher speed of lithium ion transportation.

Fig. 2 shows the pictures for the cathode material in Example 1 versus that in Comparative example 1 obtained with powder of the material under a field emission scanning electron microscope (S-4300 Shimadzu, 15 kV) . Figs. 2a to 2b show that the material in Comparative example 1 had dense secondary particles, and the particle size of the primary particles ranged from 100 to 500 nm. Figs. 2c to 2f show that the secondary particles of the material in Example 1 were composed of many primary particles of uniform particle size (100-200 nm) . A plurality of pore structures were distributed on the secondary particles in Example 1, the size of the pores being about 1 to 2 μm. These pores were formed from decomposition of the polymer to produce a gas during the sintering process.

Fig. 3 and Fig. 4 respectively show the first charge and discharge curves and the corresponding capacity differential curves for the batteries made of the cathode material in Example 1 versus that in Comparative example 1. The first charge specific capacity was 300 mAh/g and the first discharge specific capacity was 210 mAh/g for PA-LNO in Example 1, which were significantly higher than 234 mAh/g (charge specific capacity) and 184 mAh/g (discharge specific capacity) for SS-LNO in Comparative Example 1. It can be seen from the dq/dv curves in Fig. 4 that the intensity of the redox reaction of PA-LNO was significantly higher than that of SS-LNO. This is because the porous material had large contact area with the electrolyte and had more electrochemical reaction sites, contributing to higher capacity.

Fig. 5 and Fig. 6 respectively show the curves of cycling performance test (at 0.2C in Fig. 5; and at 0.5C in Fig. 6) for the batteries made of the cathode material in Example 1 and that in Example 5 versus that in Comparative Example 1. It can be seen from the figures that at both rates, the PA-LNO material in Example 1 exhibited excellent cycling performance over the SS-LNO material in Comparative example 1. This is because the dense SS-LNO material could not withstand the stress and strain caused by intercalation and deintercalation of lithium ions during cycling, resulting in structural collapse and particle damage (Fig. 8) , which led to capacity degradation. In contrast, the porous material of PA-LNO in Example 1 could well absorb the stress and strain caused by intercalation and deintercalation of lithium ions during cycling, thus ensuring excellent cycling stability. The porous material in Example 5 further stabilized the structure of the cathode material due to the doping of Co and Al. Therefore, under the test condition of a cycling test voltage of 0.05-5 C, the capacity retention rate was greater than 94%after 50 cycles of test and greater than 88%after 100 cycles of test.

Fig. 7 shows the results of cycling performance test (0.5C) for the batteries made of the cathode material in Example 6 and that in Example 7 versus that in Comparative Example 1, indicating the improvement of the cycling performance of the PA-LNO material by the doping of Al.

Fig. 8 shows the results of cycling performance test (0.5C) for the batteries made of the cathode material in Example 8 and that in Example 9 versus that in Comparative Example 1, indicating the improvement of the cycling performance of the PA-LNO material by the doping of Co.

Fig. 9 shows the results of cycling performance test (0.5C) for the batteries made of the cathode materials in Examples 10, 11, 12 and 5 versus that in Comparative Example 1, indicating the improvement of the cycling performance of the materials by the doping of Mg. It can be seen that the porous material of LiNi _0.8Co _0.13Al _0.05Mg _0.02O ₂ still exhibited a specific capacity of 157 mAh/g after 100 cycles of 0.5 C/0.5 C charging and discharging, with a capacity retention rate of as high as 98%.

The button cell which was cycled at 0.2 C for 50 cycles was disassembled under a protective atmosphere to obtain the cathode sheet having been subjected to cycling. After rinsing and drying, cathode powder was scraped from the cathode sheet and observed for particle morphology under a transmission electron microscope. Fig. 10 shows the transmission electron microscopic pictures showing the microscopic feature of the cathode material particles after 50 cycles of 0.2C cycling of the batteries made of the cathode material in Example 1 versus that in Comparative Example 1. The SS-LNO material particles in Comparative example 1 (a, b) visibly had obvious cavities and a large number of cracks, and some particles had even cracked. In contrast, despite of the occurrence of a small number of cavities, the PA-LNO material particles in Example 1 (c, d) had no microcracks and remained intact. The material synthesized with the addition of the polymer had pore structures which could well buffer the expansion and contraction of the crystal lattices and the stress and strain in the crystal grains during the charging and discharging process, inhibit occurrence of microcracks and cavities in the particles, prevent cracking of the particles, thus ensuring the intactness of the material particles during cycling and maintaining an excellent cycling stability.

It should be noted that the above examples are only intended to illustrate rather than limit the technical solutions of the embodiments of the present invention. Although the embodiments of the present invention having been described in detail with reference to the preferred examples, those skilled in the art will appreciate that the technical solutions of the embodiments of the present invention may be modified or equivalently substituted in a manner such that the modified technical solutions do not depart from the scope of the technical solutions of the embodiments of the present invention.

Claims

A lithium ion battery cathode material, wherein:

The cathode material is a lithium-metal-oxide. The cathode material is composed of spherical secondary particles comprised of primary particles with particle diameter from 100 to 200 nm, and the secondary particles has porous structureswith pore diameter of 1 to 2 μm.
The lithium ion battery cathode material according to claim 1, wherein: the cathode material has the chemical formula of Li _1+xM _1-xO ₂, wherein x=0.05-0.25, M is one or more selected from the group consisting of Ni, Co, Mn, Al, Mg, Fe, B, Ti, Cr, Ga, Zn, V, Ge, and Sn.
A method for preparing a lithium ion battery cathode material, wherein the method comprises the following steps:

S01: mixing a metal salt and a lithium salt by a molar ratio of metal ion to lithium ion of 1: (1-1.3) , adding a volatile organic acid in an amount sufficient for complexing all of the metal ions in the metal salt and the lithium salt in the raw materials, and mixing and grinding the metal salt, the lithium salt and the volatile organic acid to obtain homogeneity;

S02: mixing the ground product from S01 with an organic polymer in a ratio of 1 to 3 mL of the organic polymer to 1 g of the ground product, and grinding the mixture homogeneously.

S03: pre-sintering the ground product from S02 at 300-500℃ for 4 to 10 hours; and

S04: sintering the pre-sintered product from S03 at 750-950℃ for 6 to 15 hours, and cooling the product to obtain the desired lithium ion battery cathode material.
The method for preparing a lithium ion battery cathode material according to claim 3, wherein in S01, the metal salt includes a member selected from the group consisting of an oxalate, acetate, nitrate or sulfate salt of the element of Ni, Co, Mn, Al, Mg, Fe, Ti, Cr, Ga, Zn, V, Ge or Sn, or a combination of two or more thereof.
The method for preparing a lithium ion battery cathode material according to claim 3, wherein in S01, the lithium salt includes a member selected from the group consisting of lithium oxalate, lithium acetate, lithium nitrate, lithium sulfate, lithium hydroxide, or lithium carbonate, or a combination of two or more thereof.
The method for preparing a lithium ion battery cathode material according to claim 3, wherein in S01, the volatile organic acid includes a member selected from the group consisting of oxalic acid, tartaric acid, malic acid, citric acid, benzoic acid, salicylic acid, caffeic acid, acetic acid, propionic acid, butyric acid, pentanoic acid, or isopentanoic acid, or a combination of two or more thereof.
The method for preparing a lithium ion battery cathode material according to claim 3, wherein in S02, the organic polymer includes a member selected from the group consisting of polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 2000, polyethylene glycol 6000, or polyethylene glycol 12000, or a combination of two or more thereof.
The method for preparing a lithium ion battery cathode material according to claim 3, wherein in S03, the pre-sintering temperature is 380 to 420℃; and in S04, the sintering temperature is 800 to 850℃.
A lithium ion battery, wherein the cathode material thereof is as set forth in claims 1 to 2.
The lithium ion battery according to claim 9, wherein: the lithium ion battery has a capacity retention rate of higher than 94%after 50 cycles of testing, and of higher than 88%after 100 cycles of testing, under the test condition of a cycling test current density of 0.05-5C.